Cobalt and rare earth metals are known as valuable metals and used for various applications in industry. Cobalt is used for positive electrode materials for secondary batteries, and furthermore for superalloys (high strength heat-resistant alloys) used for e.g. jet engines for aircraft, and the like. Rare earth metals are used for fluorescent materials, negative electrode materials for nickel-hydrogen batteries, additives for magnets installed in motors, abrasives for glass substrates used for liquid crystal display panels and hard disk drives, and the like.
In recent years, energy savings have been strongly promoted, and in the automobile industry conventional gasoline-engined cars are being rapidly replaced by hybrid cars and electric cars equipped with secondary batteries using cobalt and rare earth metals. In lighting equipment, conventional fluorescent lamps are being rapidly replaced by efficient three band fluorescent lamps using rare earth metals such as lanthan, cerium, yttrium, terbium and europium. The above cobalt and rare earth metals are scarce resources, and most of them depend on imports.
Yttrium and europium have been used for fluorescent substances in cathode ray tube television sets in analog broadcasting; however, in recent years, large numbers of cathode ray tubes have been put out of use because of the transition to liquid crystal television sets. Products which have rapidly spread, such as secondary batteries and three band fluorescent lamps, can be also easily expected to cause a large amount of waste in the future as used products. Thus, cobalt and rare earth metals, scarce resources, are treated as waste without recycling of the used products, which is not preferred in terms of resource savings and resource security. Nowadays, the establishment of a method for effectively retrieving valuable metals such as cobalt and rare earth metals from such used products is strongly demanded.
Mixtures of rare earth metals such as lanthan, cerium, yttrium, terbium and europium are used for fluorescent substances used for the three band fluorescent lamps described above. Furthermore, yttrium and europium are contained in the fluorescent substances for cathode ray tubes together with a high percentage of zinc.
As a method for retrieving a specific rare earth metal from a mixture of rare earth metals, a retrieval method by a solvent extraction method from liquid in which the mixture is dissolved in an acid such as a mineral acid is generally used. For the mutual separation of rare earth metals, there is an industrial example using e.g. a phosphorus-based extraction agent, product name PC88A (manufactured by DAIHACHI CHEMICAL INDUSTRY CO., LTD.). This extraction agent, however, has phosphorus in its structure, and thus when the agent is industrially used, a high degree of wastewater treatment is required so that public waters are not contaminated by the extraction agent and deteriorated substances thereof, which move to the drainage system. Since the total volume of the extraction agent is regulated by Water Pollution Prevention Law in specific areas in Japan, its use on an industrial scale involves concerns.
A carboxylic acid-based extraction agent (e.g. 2-methyl-2-ethyl-1-heptanoic acid:neodecanoic acid) is practically used as an extraction agent not containing phosphorus. By this extraction agent, however, extraction is carried out only in a pH region equal to or higher than neutral. Therefore, when an acid solution as described above is a target, a neutralizer is required in large amounts, and there is concern about cost increases. Furthermore, the extraction ability of carboxylic acid-based extraction agents is lower than that of the above-described phosphorus-based extraction agents, and excessive equipment is required. Therefore, there is also the problem of a cost increase.
To solve such problems, an extraction agent, which has the skeleton of diglycol amic acid and is called DODGAA, has been developed (see Patent Document 1). When this extraction agent is used, however, as shown in Non-Patent Document 1, yttrium (Y), lutetium (Lu), ytterbium (Yb), thulium (Tm), erbium (Er) and holmium (Ho), which are called heavy rare earth metals, among rare earth metals have a strong tendency to be extracted together with dysprosium (Dy), terbium (Tb), gadolinium (Gd), europium (Eu) and samarium (Sm), which are called middle rare earth metals, and thus the extraction agent is not suitable for the mutual separation of rare earth metals. In addition, the extraction rates of promethium (Pm), neodym (Nd), praseodym (Pr), cerium (Ce) and lanthan (La), which are called light rare earth metals, by DODGAA are low. Especially, europium (Eu), which is produced in small quantities and is expensive, cannot be selectively retrieved from other rare earth metals. Thus, an extraction agent which is capable of mutually separating rare earth metals and further an extraction agent which is capable of efficiently extracting light rare earth metals have not been found.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No. 2007-327085
[Non-Patent Document 1] K. Shimojo, H. Naganawa, J. Noro, F. Kubota and M. Goto; Extraction behavior and separation of lanthanides with a diglycol amic acid derivative and a nitrogen-donor ligand; Anal. Sci., 23, 1427-30, 2007 December.